In England we say that to attend mass a on a particular day is a "precept of the church" and this is understood to be an obligation in terms of practising one's religion. I would say that the term "Days of obligation" naturally infers that you are talking about going to mass.

"Catholics believe that there are some days during the year that are so special that every Catholic should go to Mass to celebrate. ...... Every Sunday counts as one of those special days, and there are seven others. Sometimes they are called 'Holy Days'. Because Catholics say they ought to go to Mass, these days are sometimes called 'Days of Obligation' or 'Holy Days (Holidays) of Obligation'.
http://www.request.org.uk/main/churches/catholic/obligation0...


It is said that one has fulfulled a "holy day precept" on attending Saturday or Sunday mass.
"A Catholic who assisted at morning and evening Mass on either Saturday or Sunday would fulfill both holy-day precepts, even if the Mass formulas were of the same day. "
http://www.ewtn.com/library/liturgy/zlitur242.htm



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Note added at 25 mins (2011-01-30 17:25:08 GMT)
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This term might not be literally translatable into English so the whole phrase that it is a part of might also need to be restructured in order to facilitate the English Idiom options I have provided above.